Telephone service to buildings is provided by drop cables whereas that within buildings is provided by riser cables. Typically, cable is pulled within conduit from a manhole to a building over what may be relatively long distances. Riser cables extend upwardly from basement vaults, for example, where they are connected to incoming drop cables to upper foors where they are connected to wiring closets. The vertical distance along which a riser cable is pulled may be considerable. Accordingly, riser cables must have suitable strength characteristics. Also, building cables must be controlled with respect to size.
In order to provide suitable cable strength properties and to achieve a totally dielectric structure, metallic wires of an earlier used cable have been replaced with glass fiber, rod-like members. The rod-like members are capable of withstanding expected compressive as well as tensile loading. Compressive loading occurs when the cable tends to contract during initial shrinkage of the jacket material and during thermal cycling. However, the use of a sufficient number of glass rods to provide the cable with suitable load carrying capability causes the cable to be relatively stiff.
A further consideration is that riser cables must not add substantially to the fuel content available during a conflagration. Also, they must meet National ELectrical Code (NEC) Standards for flame resistance and smoke evolution to minimize the spread of flame and smoke from floor to floor.
Resistance to flame spread and smoke evolution also is a required property for plenum cables. In the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items appear below the drop ceiling. The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications cables including those for computers and alarm systems. Such cables extend to the wiring closet on a floor where they are connected to a riser cable.
It is not uncommon for these plenums to be continuous throughout the length and width of each floor. When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire story of the building. The fire could travel along the length of cables which are installed in the plenum. Also, smoke can be conveyed through the plenum to adjacent areas and to other stories.
Generally, a cable sheath system which encloses a core and which comprises only a conventional plastic jacket does not exhibit acceptable flame spread and smoke evolution properties for use in a riser or in a plenum. As the temperature in such a cable rises, charring of the jacket material begins. Eventually, the charred jacket begins to decompose. If the jacket char retains its integrity, it functions to insulate the core; if not, it exposes the virgin interior of the jacket including the core and other sheath system components within the jacket to elevated temperatures. The jacket and these components therewithin begin to pyrolize and emit flammable gases. These gases ignite and, because of air drafts within the plenum, burn beyond the area of flame impingement, propagating flame and evolving smoke.
As a general rule, the National Electrical Code requires that power-limited cables in plenums and risers be enclosed in metal conduits. The initial cost of metal conduits for communications cables in plenums is relatively expensive. Also, conduit is relatively inflexible and difficult to maneuver in plenums. However, the Code permits certain exceptions to this requirement provided that such cables are tested and approved by an authority such as the Underwriters Laboratories.
The problem of acceptable plenum and riser cable design is complicated somewhat by the trend to the extension of the use of optical fiber transmission media from a loop to building distribution systems. Not only must the optical fibers be protected from transmission degradation, but also they have properties which differ significantly from those of copper conductors and hence require special treatment. Light transmitting optical fibers are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent. The degradation in transmission which results from bending is known as microbending loss. This loss can occur because of shrinkage during cooling of the jacket and because of differential thermal contractions when the thermal properties of the jacket material differ significantly from those of the enclosed optical fibers.
The prior art includes a plenum cable having a core of copper conductors as shown in U.S. Pat. No. 4,284,842 which issued on Aug. 18, 1981 in the names of C. J. Arroyo, N. J. Cogelia and R. J. Darsey. The core is enclosed in a thermal core wrap material, a corrugated metallic barrier and two helically wrapped translucent tapes. The foregoing sheath system, which depends on its reflection characteristics to keep heat away from the core, it well suited to larger size copper plenum cables. However, for smaller size cables such as optical fiber cables, the use of a heat reflective metallic shield is not only expensive, but is difficult to form about the core.
The prior art also has addressed the problem of cable jackets that contribute to flame spread and smoke evolution through the use of fluoropolymers. In one prior art small size plenum cable, disclosed in application Ser. No. 626,085 filed June 29, 1984, in the names of C. J. Arroyo et al, a sheath system includes a layer of a woven material which is impregnated with a fluorocarbon resin and which encloses a core. The woven layer has an air permeability which is sufficiently low to minimize gaseous flow through the woven layer and to delay heat transfer to the core. An outer jacket of an extrudable fluoropolymer material encloses the layer of woven material.
The use of fluoropolymers, with or without underlying protective layers, for optical fiber building cable jackets requires special consideration of material properties such as crystallinity, shrinkage due to cooling after extrusion, and the magnitude of thermal expansion coefficieints to avoid detrimental effects on the optical fibers. In the absence of special precautions, the shrinkage of fluoropolymer plastic material, which is semi-crystalline, following extrusion may put the optical fiber in compression.
Still other plenum and riser cables may include wrappings of NOMEX.RTM.-KEVLAR.RTM. aramid materials or tapes which may be impregnated with silicone. However, such a wrapping may have a thickness on the order of about 0.050 inch which adds considerably to the overall cable diameter. See U.S. Pat. No. 4,595,793 which issued on June 17, 1986 in the names of C. J. Arroyo and P. D. Thomas.
What has been needed and what has not been provided by the prior art is a cable in which conventional coated optical fibers are enclosed in a relatively uncomplicated sheath system which provides suitable resistance to flame spread and smoke evolution for the optical fiber core. Such a sheath system should be one which does not add substantially to the diameter of the cable and which has suitable strength properties.